The present invention relates to the construction of an extension housing of an automotive transmission and, more particularly, to a transmission extension housing of the type that is provided with an opened front end at which the extension housing is coupled to the transmission casing, a closed rear end through which the output shaft of the transmission extends, a bearing supporting structure disposed between the opened front end and the closed rear end and supporting a bearing for at least the output shaft, and an upper opening formed in the upper wall of the extension housing for passing therethrough a connecting arm interconnecting a shift lever and a shift fork shaft.
Hitherto, either one of die-casting and ordinary casting method employing cores has been used for the production of the transmission extension housing (referred to simply as "extension housing", hereinafter) of the type described above. More specifically, in the production of the extension housing by die-casting, the bearing supporting structure which is formed by die-casting separately from the die-cast body of the extension housing is fitted into the extension housing body and secured to the latter by means of bolts or the like. According to this method, it is required to provide the extension housing body with integral bosses for bolts, resulting in a correspondingly increased weight of the extension housing. Also, since the bearing supporting structure and the extension housing body are formed as separate components and then united to each other, a troublesome and complicated work is required for obtaining a sufficiently high precision after the components are united together. On the other hand, the ordinary casting method employing cores can provide an extremely low productivity as compared with the die-casting method, although an integral casting of the extension housing body and the bearing supporting structure is possible with the ordinary casting method.
The present inventors have made studies to clarify the reasons why the die-casting which can provide a high productivity has not been used for forming the extension housing body and the bearing supporting structure integrally at a time, and have reached a conclusion that the reason is closely concerned with the relationship between the bearing supporting structure and the upper opening of the housing body provided for a connecting arm interconnecting the shift lever and the shift fork shaft. The upper opening of the extension housing body is provided for passing therethrough the connecting arm and is adapted to be closed by a hollow shift lever retainer which supports the shift lever swingably. The position of the rear edge of the upper opening is limited by various factors such as the position of the shift lever in relation to the position of driver's seat, so that it is not allowed to locate the rear edge of the upper opening too far forwardly of the ordinary position of the shift lever. On the other hand, the bearing supporting structure is formed as a vertical partition wall which extends in a vertical plane normal to the axis of the output shaft which extends through the extension housing in the longitudinal direction thereof, and is intended to support not only the bearing for the output shaft but also the bearing for the counter gear shaft as well. Also, the bearing supporting structure has to slidably support the shift fork shaft. The position of the vertical partition wall formed as the bearing supporting structure, therefore, is determined dependent on the type of transmission.
It has been said that the size of the upper opening should be as small as possible to preserve a sufficiently high mechanical strength and rigidity of the extension housing body. This naturally results in locating the forward edge of the upper opening at a position which is considerably spaced rearwardly from the position of the upper end of the vertical partition wall. This arrangement, therefore, makes it impossible to form or shape the inner space of the extension housing body rearwardly of the vertical partition wall by using a die-casting mold part which is slidable through the upper opening.